The replication mechanisms of E. coli and its single-stranded bacteriophages are being investigated by biochemical and genetic techniques. Nicked molecules of the M13 replicative form DNA have been shown to contain a specific discontinuity which is being characterized by DNA sequence analysis of the 5' terminus. The replication origins of the single-stranded phages G4 and phi X174 are being studied in the M13 genome. The specific regions of each phage DNA required for DNA replication will be determined. Similarly, the origin of replication of the E. coli chromosome will be inserted into M13 and its function analyzed. In addition, the replication terminus of the E. coli plasmid R6K will be inserted into M13 and its function investigated.